Ministers are being urged to fine schools that are informally excluding poorly performing pupils, amid mounting evidence that some institutions are attempting to game the exam system.

Hundreds of cases of children being removed from schools on tenuous and potentially illegal grounds have been reported to a charity offering legal advice to parents. Experts blame the rise of so-called “off-rolling” on schools that are under pressure to improve performance.

Children with special educational needs and disabilities (Send) are thought to be the most affected by the informal methods designed to move them out of a school without recording their departure as an official exclusion. With pressure mounting on the Department for Education to act, Anne Longfield, the children’s commissioner for England, said some schools were “abandoning their responsibility” to give a decent education to their children. She toldthe Observer it was “increasingly clear that some schools are gaming the system by taking children they think won’t get good results off their rolls before they sit their exams. Any school that does this is abandoning their responsibility to children, passing the buck to others who are often ill-equipped and don’t have the support they need to provide a good education. As a result, very vulnerable children are falling through the gaps in the education system, increasing the chances they will then go on to lead difficult adult lives.

“I will be calling on the government to set out what measures it will take, including looking at the possibility of financial penalties for schools, to ensure this practice stops now and does not become an acceptable part of the school system.”

Last week Amanda Spielman, the Ofsted chief, described off-rolling as “an invidious example of where schools have lost sight of the purpose of education”.

Richard Oldershaw, senior legal consultant of the Child Law Advice Service (CLAS), run by the charity Coram, said: “According to Department for Education data, the rate of schools exclusions has stayed roughly the same since 2010. However, evidence from CLAS suggests the number of children excluded from schools is likely to be significantly higher than this figure as unofficial or unlawful exclusions by schools are not recorded in government data.

“In the last 20 months, CLAS advised about schools exclusions in 1,704 calls. In a quarter of the calls relating to exclusions from primary schools, the adviser concluded that the school may have acted unlawfully, either by not complying with the correct procedures or because it did not adequately consider the child’s special educational needs.

“An exclusion may be necessary, if regrettable, but an exclusion must be undertaken in a fair and transparent manner.”

Research last year by the Education Datalab thinktank found that 125 schools would see GCSE pass rates drop by at least five percentage points if they included children who had left early. It identified a group of almost 20,000 children who leave the rolls of mainstream secondary schools, with only 6% recorded as achieving five good GCSEs.

The Observer has been contacted by several people claiming they are aware of cases of children being unfairly excluded in ways that are not captured by official figures. One child psychologist said they had come across an “astonishing” case in a south-east comprehensive that was the worst they had encountered in 12 years.

The student, currently living in care, was excluded on a second warning without any consultation with their support team in the first month following the takeover of a school deemed to be failing.

The student was subjected to a so-called “managed move” and shifted to a pupil referral unit, where other students at the unit were walking out of lessons and insulting teachers. Repeated attempts to meet the leadership at the pupil’s original school were ignored.

A Department for Education spokeswoman said that an independent review of exclusions was under way.

“Any decision to exclude should be lawful, reasonable and fair,” she said.

“While exclusion can be used as a sanction for schools to deal with poor behaviour, permanent exclusion should only be used as a last resort, in response to a serious breach, or persistent breaches, of the school’s behaviour policy, and where allowing the pupil to remain in school would seriously harm the education or welfare of the pupil or others.

“We will be launching an externally led review aimed at improving practice in exclusions through sharing best practice nationwide, focusing on the experiences of those groups who are disproportionately likely to be excluded.”